1. Technical Field of the Invention
This invention relates to an automatic range control method for a compact and portable optical density/dot percentage measuring device which can use an ordinary light table as its light source, digitally display a result of measurement, and has a high precision in measurement.
2. Prior Art of the Invention and its Technical Problems
An optical density meter is an instrument which detects and displays the intensities of a light source for irradiating a specimen and a transmitted or reflected light from the specimen. In general, the light transmitted from a specimen is converted into an electrical signal by a photoelectric conversion element. The electrical signal is then converted into a density value by either an analog means, or by a digital means such as a microcomputer after the electrical signal is converted from an analog signal into a digital signal. The density value is thereafter displayed. A dot percentage meter is an instrument which measures a gradation (collective density) reproduced from a change in dot areas, based upon the measurement of transmittance or reflectance of a specimen to be measured. Optical density meters and dot percentage meters may be used in common if a different calculation method for display values is used for the respective meters. First, an optical density meter will be explained and thereafter a dot percentage meter will be explained. It is necessary for an optical density meter to measure the quantity of light over a wide optical density range. Therefore, a programmable gain amplifier is provided between a photoelectric conversion element and an A/D converter to amplify a small input signal and thereafter convert the amplified analog signal into a digital signal. This is one of the methods to ensure the precision of A/D conversion.
FIG. 4 is a block diagram showing an example of such an optical density measuring device. A system using a sensor, programmable gain amplifier, A/D converter and microcomputer has been well known in the art. For instance, there is known an SBC711 A/D converter board manufactured by Intel Corp. A system using the SBC711 A/D converter board comprises an analog multiplexer, programmable gain amplifier, sample/hold circuit, A/D converter and CPU. By removing the analog multiplexer and sample/hold circuit and adding a light source, photoelectric conversion element 11, pre-amplifier 12 and display 16, a conventional optical density measuring device as shown in FIG. 4 can be constructed by using the above system available in the market. Assume that the density measuring range of the optical density measuring device constructed as above is 0.0 to 4.0, the gain switching range of the programmable gain amplifier 13 becomes .times.1 to .times.1000. With such a wide range of gain switching, usually an off-set output voltage of the preamplifier 12 is also amplified at the gain amplifier 13. Therefore, to ensure a highly precise measurement, the preamplifier 12 is needed to be of a high precision thereby resulting in an expensive preamplifier 12.
FIG. 5 is a circuit diagram showing an example of a conventional range switching circuit. The above-mentioned disadvantage of the optical density measuring device is also related to a disadvantage accompanied by an illuminometer while its range is switched. One of the known methods to solve this problem is to use the range switching circuit of FIG. 5, which is disclosed in "Practical Electronic Circuit Handbook" (Published October, 1975, by CQ Publishing Co., Ltd.). The range switching of the circuit is performed by changing the value of a feedback resistor 17 of a preamplifier 12 by means of a range switch 18, without amplifying an off-set voltage of the preamplifier 12. However, a bias current of the preamplifier 12 is amplified so that an amplifier having input FET transistors and a small bias current should be used as the preamplifier 12.
FIG. 6 is a circuit diagram showing an example of a conventional optical density measuring device to which the range switching circuit shown in FIG. 5 is applied. This conventional device is the optical density meter disclosed in Japanese Unexamined Patent Publication No. 56-79946, which comprises a photoelectric conversion element 11, operational amplifier 19, A/D converter 20, operation processing circuit 21, display 22, feedback resistor group 23 (23a to 23d) and analog switch group 24 (24a to 24d). In this case, however, an analog switch is not ideal but has a leakage current even at an off state and produces some error in measurement. This error in measurement essentially increases in proportion to the number of switches. In order to reduce the error in measurement, an analog switch having an ideal characteristic is needed which has no leakage current and zero on-resistance as well as infinite off-resistance. To this end, however, it is necessary to use an expensive analog switch. As a result, the number of analog switches used with an optical density measuring device should preferably be as small as possible in view of improvement on measuring precision, miniaturization and low cost of the circuit.
Apart from the above problems, most of conventional optical density/dot percentage meters have been of a fixed mount type and used a stable light source whose flicker or variation can be neglected by using a light source such as a halogen lamp powered with a stabilized power source. Therefore, those conventional meters are not suitable for using a common commercial light table as their light source and cannot be used as portable optical density measuring devices or dot percentage measuring devices because of a large number of circuit elements and large outer dimension.
Of the dot percentage measuring device of this type, there is known a dot percentage measuring meter developed by the present applicant aiming at measuring a dot percentage of such as a print original film, which is shown in FIG. 9. This meter has been developed in place of the conventional fixed mount type meter and is featured by its compact and portable nature. This dot percentage meter can accordingly be used for measurement at any location so long as a light table is available.
However, since the light table is fixedly mounted, the dot percentage meter cannot always make the best use of its portable nature and the location of measurement is limited.